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By the end of this section, you will be able to:
  • Derive the predicted ground-state electron configurations of atoms
  • Identify and explain exceptions to predicted electron configurations for atoms and ions
  • Relate electron configurations to element classifications in the periodic table

Having introduced the basics of atomic structure and quantum mechanics, we can use our understanding of quantum numbers to determine how atomic orbitals relate to one another. This allows us to determine which orbitals are occupied by electrons in each atom. The specific arrangement of electrons in orbitals of an atom determines many of the chemical properties of that atom.

Orbital energies and atomic structure

The energy of atomic orbitals increases as the principal quantum number, n , increases. In any atom with two or more electrons, the repulsion between the electrons makes energies of subshells with different values of l differ so that the energy of the orbitals increases within a shell in the order s < p < d < f. [link] depicts how these two trends in increasing energy relate. The 1 s orbital at the bottom of the diagram is the orbital with electrons of lowest energy. The energy increases as we move up to the 2 s and then 2 p , 3 s , and 3 p orbitals, showing that the increasing n value has more influence on energy than the increasing l value for small atoms. However, this pattern does not hold for larger atoms. The 3 d orbital is higher in energy than the 4 s orbital. Such overlaps continue to occur frequently as we move up the chart.

A table entitled, “Subshell electron capacity,” is shown. Along the left side of the table, an upward pointing arrow labeled, “E,” is drawn. The table includes three columns. The first column is narrow and is labeled, “2.” The second is slightly wider and is labeled, “6.” The third is slightly wider yet and is labeled, “10.” The fourth is the widest and is labeled, “14.” The first column begins at the very bottom with a horizontal line segment labeled “1 s.” Evenly spaced line segments continue up to 7 s near the top of the column. In the second column, a horizontal dashed line segment labeled, “2 p,” appears at a level between the 2 s and 3 s levels. Similarly 3 p appears at a level between 3 s and 4 s, 4 p appears just below 5 s, 5 p appears just below 6 s, and 6 p appears just below 7 s. In the third column, a dashed line labeled, “3 d,” appears just below the level of 4 p. Similarly, 4 d appears just below 5 p and 5 d appears just below 6 p. Six d however appears above the levels of both 6 p and 7 s. The far right column entries begin with a dashed line labeled, “4 f,” positioned at a level just below 5 d. Similarly, a second dashed line segment appears just below the level of 6 d, which is labeled, “5 f.”
Generalized energy-level diagram for atomic orbitals in an atom with two or more electrons (not to scale).

Electrons in successive atoms on the periodic table tend to fill low-energy orbitals first. Thus, many students find it confusing that, for example, the 5 p orbitals fill immediately after the 4 d , and immediately before the 6 s . The filling order is based on observed experimental results, and has been confirmed by theoretical calculations. As the principal quantum number, n , increases, the size of the orbital increases and the electrons spend more time farther from the nucleus. Thus, the attraction to the nucleus is weaker and the energy associated with the orbital is higher (less stabilized). But this is not the only effect we have to take into account. Within each shell, as the value of l increases, the electrons are less penetrating (meaning there is less electron density found close to the nucleus), in the order s > p > d > f . Electrons that are closer to the nucleus slightly repel electrons that are farther out, offsetting the more dominant electron–nucleus attractions slightly (recall that all electrons have −1 charges, but nuclei have + Z charges). This phenomenon is called shielding and will be discussed in more detail in the next section. Electrons in orbitals that experience more shielding are less stabilized and thus higher in energy. For small orbitals (1 s through 3 p ), the increase in energy due to n is more significant than the increase due to l ; however, for larger orbitals the two trends are comparable and cannot be simply predicted. We will discuss methods for remembering the observed order.

Questions & Answers

the first fifty elements
Joy Reply
how to complete and balance ionic equation?
BARAKA Reply
what is molar mass?
Wendy Reply
Molar mass is defined as the mass of a sample of a compound divided by the amount of substance of that sample.
Love
right
xavi
or M=m\n mathematically.
xavi
atom is the smallest indivisible part of an element that takes place in chemical reaction
Obinna Reply
An atom is the smallest indivisible part of an element
Vibes
what is periodic table
Faridi
what is chemistry?
Mathias Reply
what is an atom
Mathias
what is an atom?
Mathias
is the study of matter and the changes it under goes.
xavi
under goes through three state they are solid liquid and gas
Samuel
in a sturctural way formula for C6H12
Samuel
sorry ,it's properties and structures
xavi
please what is hydroxyl group
Gold Reply
the hydroxyl group is a functional group consisting of a hydrogen atom covalently bonded to an oxygen atom.
Itz
Names are of homologous series
Josiah Reply
what's an hydroxyl group?
VICTORY Reply
A hydroxy or hydroxyl group is the entity with the formula OH. It contains oxygen bonded to hydrogen. In organic chemistry, alcohols and carboxylic acids contain hydroxy groups. The anion [OH⁻], called hydroxide, consists of a hydroxyl group. 
okwara
what are the steps to follow when writing the condensed formula of elements
Jessicafaith Reply
What is organic chemistry
John Reply
what's organic chemistry?
Mr Reply
why do electrons have no mass
Skynetic Reply
hello
Samuel
hello there
Oliver
hi and who say electrons has no mass ? it does have but most at time consider negleable
Eyong
hello
Lagazani
please what is maa
Lagazani
yeah i will calculate it
divine
Because it's mass it very very small compare with mass of atom
Yasmeen
it's simple
lola
consider the rxn of butadiene .Butadiene is prepared by the gas phase catalytic dehydrogenation of 1-butene , at 900K and 1 bar. c4H8 (g)_ C4H6 (g) + H2 (g) (a) In order to suppress side reactions , the butane is diluted with steam before it passes into the reactor.Estimate the conversion
Rodrick
Estimate the conversion of 1-butene for a feed consisting of 10 moles of steam per mole of 1-butene. (b) find the conversion if the inerts were absent and side reactions are ignored .
Rodrick
help me please I have tried
Rodrick
wat is the difference between electrochemical chemical corrosion
Uzor
please how can i solve this project work..modern aid science process and product
Lagazani
what is the definition of chemistry
Sophia Reply
Sophia pls check on fb u will see transcendence Internationale is a group from here just join over, and ask any question in science from there we here created it
Eyong
the icon is clear white then writing transcendence Internationale study group
Eyong
U will see it
Eyong
for now chemistry is the study of matter and it properties, why and how element combined to form other substances
Eyong
Chemistry is a group that identifies properties of matter and many more
Alona
Wow and chemistry also is the systematic study of nature
Eyong
chemistry is the study of the structure, properties of matter and the effect of addition or removal of heat from substances.
Bra
calculate the relative molecular Mass of the following 1 ZnCl2 2 NaNo3 3 HNO3 4 CaO
Reuben Reply
2) 23+14+(16×3)= 3)1+14+(16×3)= 4) 40+16= 1) Zn+71= that's how they do it but for Zn I don't know its mass number
Eyong
Reuben u are welcome to join the group transcendence Internationale on fb we created it for science students is a question and answering platform for all science subjects
Eyong
yeah it's fun
Christian
hi guys I'm new here, please I need definition of acid and base
Gold
please some one should tell me the meaning of hydro carbon
Glory
What is radioactivity
Aaron Reply
radioactivity is the spontaneous disintegration of the nucleus of an atom of an element to emmit radiation
Vivian
Practice Key Terms 7

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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